Some construction and build notes for the deployed prototypes.
We recommend having all hardware connected directly to the Raspberry Pi, including power connectors. An expansion board like this one or this one will greatly simplify connecting devices to the I2C bus and power pins.
Through trial-and-error, we determined the Alphasense OPC-N2 aerosol sensor exhibits high sensitivity to input voltage and ground differentials between its data and power lines. To avoid the dreaded "your firmware could not be detected" error, connect power input and power ground lines directly to the Pi Zero's 5V and G rails, respectively.
The Pi can source up to 1.5A through its GPIO pins, which is sufficient to support the OPC-N2 start-up current requirements. Of course, you should use a high quality power supply that can source at least 2A.
Diagram of wiring connections (.png is embedded with Draw.io xml file):
To-scale CAD drawings of environmental sensors for positioning enclosure cut-outs and mounting holes. PDFs sized for Letter paper.
- CO2 sensor (K30): .dxf, .pdf
- PM sensor (OPC-N2): .dxf, .pdf
- T/RH sensor (HTU21D): .dxf, .pdf
- P/T sensor (BMP280): n/a, mounted inside enclosure
Sensors are exposed to ambient conditions through holes in the bottom of the enclosure. See CAD Drawings above for printable position templates.
The pressure/temperature sensor is located entirely within the enclosure. It still represents barometric pressure as the box is not air-tight, but the temperature measurement does not reflect ambient conditions.
Rudimentary weatherproofing of the T/RH sensor (HTU21D) is done with hot glue:
The aerosol sensor (OPC-N2) has a foam gasket for weather resistance; secure it using M3 screws (try 5mm length). The CO2 sensor (K30) fits snugly to the enclosure on #4-40, 5/16" tall stand-offs:
Raspberry Pi, pressure/temperature sensor (BMP280) and real-time clock (RTC) (DS3231) are mounted to the rear panel:
Keep those connectors (all of them!) in place with hot glue:
You can improvise stand-offs of any size using long bolts and hex nuts. This photo also shows a more compact Pi Hat layout with clock and P/T sensor directly mounted.
The inside of an assembled package looks like this:
And the three prototype units looked like this during their initial bench tests on the Paccar roof Dec 2016-Jan 2017.
